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Does an increase in the 100-year flood estimate originate from climate or land-use change?

Climate variability (dry cycles to wet cycles) and land-use change play a significant role, but there is a large amount of uncertainty around the flood quantile estimates (the value of discharge corresponding to the 100-year flood), particularly if there isn’t a long record of observed data at a stream location.  

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Picture of tow truck driver assisting car stuck in flood waters.

How are floods predicted?

Flood predictions require several types of data:The amount of rainfall occurring on a real-time basis.The rate of change in river stage on a real-time basis, which can help indicate the severity and immediacy of the threat.Knowledge about the type of storm producing the moisture, such as duration, intensity and areal extent, which can be valuable for determining possible severity of the flooding...

link

How are floods predicted?

Flood predictions require several types of data:The amount of rainfall occurring on a real-time basis.The rate of change in river stage on a real-time basis, which can help indicate the severity and immediacy of the threat.Knowledge about the type of storm producing the moisture, such as duration, intensity and areal extent, which can be valuable for determining possible severity of the flooding...

Learn More
link
Historic photo of flooding in Hazard, KY

How can a 1,000-year rainfall not result in a 1,000-year flood?

It comes down to a number of factors, including the pattern of movement of the rainstorm in each particular watershed, the conditions of the soil and plant matter in the watershed, and the timing of the rainstorm in one watershed versus other watersheds. For example, if the ground is already saturated before a rainstorm, much of the rain will run off into streams, but if the ground is dry, it will...

link

How can a 1,000-year rainfall not result in a 1,000-year flood?

It comes down to a number of factors, including the pattern of movement of the rainstorm in each particular watershed, the conditions of the soil and plant matter in the watershed, and the timing of the rainstorm in one watershed versus other watersheds. For example, if the ground is already saturated before a rainstorm, much of the rain will run off into streams, but if the ground is dry, it will...

Learn More
link
Poster: 100-Year Flood--It's All About Chance

We had a "100-year flood" two years in a row. How can that be?

The term "100-year flood" is used to describe the recurrence interval of floods. The 100-year recurrence interval means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year...

link

We had a "100-year flood" two years in a row. How can that be?

The term "100-year flood" is used to describe the recurrence interval of floods. The 100-year recurrence interval means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year...

Learn More
link
Image: Flooding in Waverly, IA

Why do the values for the 100-year flood seem to change with every flood?

The amount of water corresponding to a 100-year flood, a 500-year flood, or a 1,000-year flood is known as a "flood quantile". For instance, on a given river, the flood quantile corresponding to the 50-year flood might be 10,000 cubic feet per second (cfs) and the flood quantile corresponding to the 100-year flood might be 15,000 cfs. The estimates of the flood quantiles are calculated using...

link

Why do the values for the 100-year flood seem to change with every flood?

The amount of water corresponding to a 100-year flood, a 500-year flood, or a 1,000-year flood is known as a "flood quantile". For instance, on a given river, the flood quantile corresponding to the 50-year flood might be 10,000 cubic feet per second (cfs) and the flood quantile corresponding to the 100-year flood might be 15,000 cfs. The estimates of the flood quantiles are calculated using...

Learn More
link
Hattiesburg Flood Inundation Map

Where can I find flood maps?

FEMA is the official public source for flood maps for insurance purposes:FEMA’s Flood Map Service CenterFEMA’s Flood Hazard Map FAQs NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data:National Flood ForecastsInteractive Flood Information MapCoastal Inundation Dashboard: Real-time and historical coastal flooding informationUSGS flood map...

link

Where can I find flood maps?

FEMA is the official public source for flood maps for insurance purposes:FEMA’s Flood Map Service CenterFEMA’s Flood Hazard Map FAQs NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data:National Flood ForecastsInteractive Flood Information MapCoastal Inundation Dashboard: Real-time and historical coastal flooding informationUSGS flood map...

Learn More
link
Image: Flooding in Finchfield, IA

What is a 1,000-year flood?

The term “1,000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year. These statistical values are based on observed data.

link

What is a 1,000-year flood?

The term “1,000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year. These statistical values are based on observed data.

Learn More
link
Flooded roadway

What are the two types of floods?

There are two basic types of floods: flash floods and the more widespread river floods. Flash floods generally cause greater loss of life and river floods generally cause greater loss of property.A flash flood occurs when runoff from excessive rainfall causes a rapid rise in the water height (stage) of a stream or normally-dry channel. Flash floods are more common in areas with a dry climate and...

link

What are the two types of floods?

There are two basic types of floods: flash floods and the more widespread river floods. Flash floods generally cause greater loss of life and river floods generally cause greater loss of property.A flash flood occurs when runoff from excessive rainfall causes a rapid rise in the water height (stage) of a stream or normally-dry channel. Flash floods are more common in areas with a dry climate and...

Learn More
man in yellow slicker standing in rushing river up to his thighs, holding a stick-like instrument in the water
link
Video - When rivers rise: warning you before the next flood (WCET TV, N.C.)
link
video thumbnail: The Anatomy of Floods: The Causes and Development of 2011's Epic Flood Events
link
flood sign
link
link
video thumbnail: Interview: 2009 Fargo, ND Flooding
link
USGS CoreCast
link
USGS
link
Poster: 100-Year Flood--It's All About Chance
link
Image: Flooding of the Sorlie Bridge
link
Reading the staff gage during flooding in Las Vegas, Nev.
link
A photo of floodwaters reaching up to a street sign in Texas
link

Guidelines for determining flood flow frequency—Bulletin 17C

Accurate estimates of flood frequency and magnitude are a key component of any effective nationwide flood risk management and flood damage abatement program. In addition to accuracy, methods for estimating flood risk must be uniformly and consistently applied because management of the Nation’s water and related land resources is a collaborative effort involving multiple actors including most level
By
Water Resources, Groundwater and Streamflow Information Program

Development of flood regressions and climate change scenarios to explore estimates of future peak flows

A new Web-based application, titled “Application of Flood Regressions and Climate Change Scenarios To Explore Estimates of Future Peak Flows”, has been developed by the U.S. Geological Survey, in cooperation with the New York State Department of Transportation, that allows a user to apply a set of regression equations to estimate the magnitude of future floods for any stream or river in New York S
By
Water Resources, New York Water Science Center

Popular myths about flooding in Western Washington

Floods are the most destructive natural hazard in the Nation, causing more deaths and financial loss in the 20th century than any other natural disaster. The most significant 20 riverine floods of the 20th century for which data are available have killed more than 1,843 people and caused more than $50 billion (uninflated) in damages (Perry, 2000). One of the most common means of describing the sev
By
Water Resources, Washington Water Science Center

Climate change, atmospheric rivers, and floods in California - a multimodel analysis of storm frequency and magnitude changes

Recent studies have documented the important role that “atmospheric rivers” (ARs) of concentrated near‐surface water vapor above the Pacific Ocean play in the storms and floods in California, Oregon, and Washington. By delivering large masses of warm, moist air (sometimes directly from the Tropics), ARs establish conditions for the kinds of high snowlines and copious orographic rainfall that have

100-Year flood–it's all about chance

In the 1960's, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has a
By
Water Resources, Kansas Water Science Center

The world's largest floods, past and present: Their causes and magnitudes

Floods are among the most powerful forces on earth. Human societies worldwide have lived and died with floods from the very beginning, spawning a prominent role for floods within legends, religions, and history. Inspired by such accounts, geologists, hydrologists, and historians have studied the role of floods on humanity and its supporting ecosystems, resulting in new appreciation for the many-fa
By
Water Resources, Natural Hazards, Volcano Hazards, Volcano Science Center

Large floods in the United States: where they happen and why

The spatial distribution of large gaged floods throughout the United States shows that the locations of most of the largest flows are related to specific combinations of regional climatology, topography, and basin size. Key factors include the general northward trend of decreasing atmospheric moisture, proximity to oceanic moisture sources such as the Pacific Ocean and the Gulf of Mexico, and orie
By
Water Resources, Natural Hazards, Volcano Hazards, Volcano Science Center, Kansas Water Science Center

Floods and flood plains

The "100-Year Flood"

By
Kansas Water Science Center, Washington Water Science Center
link

New US Geological Survey-led Research Helps California Coastal Managers Prioritize Planning and Mitigation Efforts Due to Rising Seas and Storms

New U.S. Geological Survey-led coastal modeling research presents state, federal, and commercial entities with varying storm and sea level-rise...

Read Article
link

USGS Science – Leading the Way for Preparedness

Learn About USGS Hazards Science and More About National Preparedness Month: The very nature of natural hazards means that they have the potential to...

Read Article
link

Study Links Major Floods in North America and Europe to Multi-Decade Ocean Patterns

The number of major floods in natural rivers across Europe and North America has not increased overall during the past 80 years, a recent study has...

Read Article
link

Stormy weather: How the USGS goes to work monitoring its effects

Atmospheric rivers are a global weather phenomenon that can bring large amounts of rain or snow to the U.S. West Coast each year. These rivers of wet...

Read Article
link

Fragmented Patterns Seen in the Recent History of U.S. Floods

Some regional trends; no widespread national pattern

Read Article

Related Content

  • FAQ
    link
    Picture of tow truck driver assisting car stuck in flood waters.

    How are floods predicted?

    Flood predictions require several types of data:The amount of rainfall occurring on a real-time basis.The rate of change in river stage on a real-time basis, which can help indicate the severity and immediacy of the threat.Knowledge about the type of storm producing the moisture, such as duration, intensity and areal extent, which can be valuable for determining possible severity of the flooding...

    link

    How are floods predicted?

    Flood predictions require several types of data:The amount of rainfall occurring on a real-time basis.The rate of change in river stage on a real-time basis, which can help indicate the severity and immediacy of the threat.Knowledge about the type of storm producing the moisture, such as duration, intensity and areal extent, which can be valuable for determining possible severity of the flooding...

    Learn More
    link
    Historic photo of flooding in Hazard, KY

    How can a 1,000-year rainfall not result in a 1,000-year flood?

    It comes down to a number of factors, including the pattern of movement of the rainstorm in each particular watershed, the conditions of the soil and plant matter in the watershed, and the timing of the rainstorm in one watershed versus other watersheds. For example, if the ground is already saturated before a rainstorm, much of the rain will run off into streams, but if the ground is dry, it will...

    link

    How can a 1,000-year rainfall not result in a 1,000-year flood?

    It comes down to a number of factors, including the pattern of movement of the rainstorm in each particular watershed, the conditions of the soil and plant matter in the watershed, and the timing of the rainstorm in one watershed versus other watersheds. For example, if the ground is already saturated before a rainstorm, much of the rain will run off into streams, but if the ground is dry, it will...

    Learn More
    link
    Poster: 100-Year Flood--It's All About Chance

    We had a "100-year flood" two years in a row. How can that be?

    The term "100-year flood" is used to describe the recurrence interval of floods. The 100-year recurrence interval means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year...

    link

    We had a "100-year flood" two years in a row. How can that be?

    The term "100-year flood" is used to describe the recurrence interval of floods. The 100-year recurrence interval means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year...

    Learn More
    link
    Image: Flooding in Waverly, IA

    Why do the values for the 100-year flood seem to change with every flood?

    The amount of water corresponding to a 100-year flood, a 500-year flood, or a 1,000-year flood is known as a "flood quantile". For instance, on a given river, the flood quantile corresponding to the 50-year flood might be 10,000 cubic feet per second (cfs) and the flood quantile corresponding to the 100-year flood might be 15,000 cfs. The estimates of the flood quantiles are calculated using...

    link

    Why do the values for the 100-year flood seem to change with every flood?

    The amount of water corresponding to a 100-year flood, a 500-year flood, or a 1,000-year flood is known as a "flood quantile". For instance, on a given river, the flood quantile corresponding to the 50-year flood might be 10,000 cubic feet per second (cfs) and the flood quantile corresponding to the 100-year flood might be 15,000 cfs. The estimates of the flood quantiles are calculated using...

    Learn More
    link
    Hattiesburg Flood Inundation Map

    Where can I find flood maps?

    FEMA is the official public source for flood maps for insurance purposes:FEMA’s Flood Map Service CenterFEMA’s Flood Hazard Map FAQs NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data:National Flood ForecastsInteractive Flood Information MapCoastal Inundation Dashboard: Real-time and historical coastal flooding informationUSGS flood map...

    link

    Where can I find flood maps?

    FEMA is the official public source for flood maps for insurance purposes:FEMA’s Flood Map Service CenterFEMA’s Flood Hazard Map FAQs NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data:National Flood ForecastsInteractive Flood Information MapCoastal Inundation Dashboard: Real-time and historical coastal flooding informationUSGS flood map...

    Learn More
    link
    Image: Flooding in Finchfield, IA

    What is a 1,000-year flood?

    The term “1,000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year. These statistical values are based on observed data.

    link

    What is a 1,000-year flood?

    The term “1,000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year. These statistical values are based on observed data.

    Learn More
    link
    Flooded roadway

    What are the two types of floods?

    There are two basic types of floods: flash floods and the more widespread river floods. Flash floods generally cause greater loss of life and river floods generally cause greater loss of property.A flash flood occurs when runoff from excessive rainfall causes a rapid rise in the water height (stage) of a stream or normally-dry channel. Flash floods are more common in areas with a dry climate and...

    link

    What are the two types of floods?

    There are two basic types of floods: flash floods and the more widespread river floods. Flash floods generally cause greater loss of life and river floods generally cause greater loss of property.A flash flood occurs when runoff from excessive rainfall causes a rapid rise in the water height (stage) of a stream or normally-dry channel. Flash floods are more common in areas with a dry climate and...

    Learn More
  • Multimedia
    man in yellow slicker standing in rushing river up to his thighs, holding a stick-like instrument in the water
    link
    Video - When rivers rise: warning you before the next flood (WCET TV, N.C.)
    link
    video thumbnail: The Anatomy of Floods: The Causes and Development of 2011's Epic Flood Events
    link
    flood sign
    link
    link
    video thumbnail: Interview: 2009 Fargo, ND Flooding
    link
    USGS CoreCast
    link
    USGS
    link
    Poster: 100-Year Flood--It's All About Chance
    link
    Image: Flooding of the Sorlie Bridge
    link
    Reading the staff gage during flooding in Las Vegas, Nev.
    link
    A photo of floodwaters reaching up to a street sign in Texas
    link
  • Publications

    Guidelines for determining flood flow frequency—Bulletin 17C

    Accurate estimates of flood frequency and magnitude are a key component of any effective nationwide flood risk management and flood damage abatement program. In addition to accuracy, methods for estimating flood risk must be uniformly and consistently applied because management of the Nation’s water and related land resources is a collaborative effort involving multiple actors including most level
    By
    Water Resources, Groundwater and Streamflow Information Program

    Development of flood regressions and climate change scenarios to explore estimates of future peak flows

    A new Web-based application, titled “Application of Flood Regressions and Climate Change Scenarios To Explore Estimates of Future Peak Flows”, has been developed by the U.S. Geological Survey, in cooperation with the New York State Department of Transportation, that allows a user to apply a set of regression equations to estimate the magnitude of future floods for any stream or river in New York S
    By
    Water Resources, New York Water Science Center

    Popular myths about flooding in Western Washington

    Floods are the most destructive natural hazard in the Nation, causing more deaths and financial loss in the 20th century than any other natural disaster. The most significant 20 riverine floods of the 20th century for which data are available have killed more than 1,843 people and caused more than $50 billion (uninflated) in damages (Perry, 2000). One of the most common means of describing the sev
    By
    Water Resources, Washington Water Science Center

    Climate change, atmospheric rivers, and floods in California - a multimodel analysis of storm frequency and magnitude changes

    Recent studies have documented the important role that “atmospheric rivers” (ARs) of concentrated near‐surface water vapor above the Pacific Ocean play in the storms and floods in California, Oregon, and Washington. By delivering large masses of warm, moist air (sometimes directly from the Tropics), ARs establish conditions for the kinds of high snowlines and copious orographic rainfall that have

    100-Year flood–it's all about chance

    In the 1960's, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has a
    By
    Water Resources, Kansas Water Science Center

    The world's largest floods, past and present: Their causes and magnitudes

    Floods are among the most powerful forces on earth. Human societies worldwide have lived and died with floods from the very beginning, spawning a prominent role for floods within legends, religions, and history. Inspired by such accounts, geologists, hydrologists, and historians have studied the role of floods on humanity and its supporting ecosystems, resulting in new appreciation for the many-fa
    By
    Water Resources, Natural Hazards, Volcano Hazards, Volcano Science Center

    Large floods in the United States: where they happen and why

    The spatial distribution of large gaged floods throughout the United States shows that the locations of most of the largest flows are related to specific combinations of regional climatology, topography, and basin size. Key factors include the general northward trend of decreasing atmospheric moisture, proximity to oceanic moisture sources such as the Pacific Ocean and the Gulf of Mexico, and orie
    By
    Water Resources, Natural Hazards, Volcano Hazards, Volcano Science Center, Kansas Water Science Center

    Floods and flood plains

    The "100-Year Flood"

    By
    Kansas Water Science Center, Washington Water Science Center
  • News
    link

    New US Geological Survey-led Research Helps California Coastal Managers Prioritize Planning and Mitigation Efforts Due to Rising Seas and Storms

    New U.S. Geological Survey-led coastal modeling research presents state, federal, and commercial entities with varying storm and sea level-rise...

    Read Article
    link

    USGS Science – Leading the Way for Preparedness

    Learn About USGS Hazards Science and More About National Preparedness Month: The very nature of natural hazards means that they have the potential to...

    Read Article
    link

    Study Links Major Floods in North America and Europe to Multi-Decade Ocean Patterns

    The number of major floods in natural rivers across Europe and North America has not increased overall during the past 80 years, a recent study has...

    Read Article
    link

    Stormy weather: How the USGS goes to work monitoring its effects

    Atmospheric rivers are a global weather phenomenon that can bring large amounts of rain or snow to the U.S. West Coast each year. These rivers of wet...

    Read Article
    link

    Fragmented Patterns Seen in the Recent History of U.S. Floods

    Some regional trends; no widespread national pattern

    Read Article